The present invention concerns a process for detecting the zero crossings of a data signal by means of a comparator.
Such a process is disclosed, for example, in the book by U. Tietze and Ch. Schenk "Halbleiter-Schaltungstechnik" Semiconductor Circuitry 3.sup.rd edition, 1976, pages 296 and 297. In this known process, a difference amplifier receives a reference voltage at one input and at another input it receives an input voltage to be monitored. In this known process, a maximum voltage appears at the output of the difference amplifier when the input voltage is higher than the reference voltage, and a minimum output voltage occurs when the input voltage is lower than the reference voltage, so a zero crossing of the input voltage can be detected due to the change in output voltage if the reference voltage is selected accordingly.
The aforementioned process does not lend itself to accurate detection of zero crossings of a data signal if the data signal has been transmitted over a system with filtering characteristics. Such a system exists, for example, with symmetrical lines in communications technology, where transformers are normally used. The transformers result in high-pass characteristics. As a result, the individual pulses transmitted have a rather marked pulse tilt even in the case of data signals with a constant signal level. The full swing of the next pulse of the data signals begins at the end of the pulse tilt of each individual pulse of the data signal, so it is impossible to accurately detect the zero crossing with a comparator having a fixed threshold because of the significant rise time of the individual pulses of the data signal. This then results in a phase jitter that is random because it depends on the data content. This is not negligible in accurate measurements and therefore must be suppressed.